5 24 THE POPULAR SCIENCE MONTHLY. 



Hopkins, published in 1847, was that the disastrous results of 

 earthquakes were caused by the velocity of the wave of translation, 

 and that theory is probably accepted by many who will distrust the 

 conclusions of Mr. Mallet. But it is obvious to every observer that 

 the enormous velocity of 1,200 miles an hour is not communicated to 

 objects on the surface as the wave passes. They are rarely thrown 

 to any considerable distance. Buildings are overthrown, but they fall 

 where they stood. 



We have already remarked that objects standing directly on the 

 uniform strata are seldom injured by earthquake-shocks. Such was 

 the case, as we have seen, with that portion of Lisbon which was built 

 on the limestone and basalt. But where the surface, perhaps hundreds 

 of feet deep, is of loose unelastic material, the transit-wave, with its vi- 

 brations, in passing through it, becomes broken into oscillations, its 

 force is dissipated and motion reduced, but the vibratory swing which 

 it communicates is sufficient to fissure the earth's surface and strew it 

 with ruins. 



On the coast of Dublin Bay, Mallet exploded gunpowder buried 

 several feet beneath the surface, in the sand, and ascertained the in- 

 tensity and velocity of the shock by a delicate seisometer. Other ex- 

 periments gave the rate and intensity of movement in more compact 

 formations with the following results : In sand, 824.9 feet in a second; 

 in divided granite, 1,306.4 feet; in compact granite, 1,664.6 feet. It 

 is found by observation that objects, as walls and chimneys, fall back- 

 ward or forward, but generally in a line with the direction in which 

 the wave travels, while fractures of walls occur in a line transverse to 

 the direction of the wave. And by diagram, Fig. 10, it will be obvious 

 that, given the position of the ruins over an extended area, not only 

 the centre but depth of disturbance may be ascertained. If the focus be 

 C, as shown in the diagram, the wave would reach the surface at an 

 angle other than if the focus be at A, and the result would appear in 

 the manner of displacement on the surface. By the principle here 

 indicated, Mallet was able to locate the central ai-ea and depth of the 

 Neapolitan earthquake of 1857, and states that the great fissure, the 

 forming of which caused the first shock, was 7| miles in length, and 

 5f miles in average depth. 



The filling of the fissure with water, and its conversion into 

 superheated steam, may have produced the subsequent shocks. By 

 calculation, the same author shows the enormous pressure and rend- 

 ing power of steam if admitted Avithout limit into such a fissure. " If 

 the temperature increase 1 Fahr. for 60 feet depth, then, at the focal 

 centre of the fissure, the temperature would be 883.4 Fahr., and the 

 pressure on the walls of the cavity not less than 640,528,000,000 tons. 

 But the pressure would be vastly increased if the temperature be near 

 that of melted rock." That this may be the case is rendered probable 

 from recent investigations of Mallet, by which he shows that the heat 



